This invention relates to a noncircular cross-section carbon fiber, a process for producing the carbon fiber and a composite of the carbon fiber with a resin, and more particularly, a noncircular cross-section carbon fiber which can give an excellent reinforcing effect to a composite of the carbon fiber with a resin, a process for producing such carbon fiber, and a composite of such carbon fiber with a resin.
Because carbon fiber is excellent in both specific strength and specific modulus when compared with other fibers, it is extensively used in the industry as a reinforcing fiber for a composite with resin by virtue of its excellent mechanical properties. Recently, the usefulness of a carbon fiber composite is increasing, and it is demanded that the performances of carbon fiber for especially sports, aircraft and aerospace uses be further heightened.
To meet such a demand for heightening the performances of carbon fiber, a marked progress was made in the properties of carbon fiber itself such as strength, modulus, etc., of a resin-impregnated strand. Although the performances of carbon fiber itself were thus improved, the properties of a composite in which such carbon fiber is actually used cannot yet be satisfactorily improved. That is, the availability of the strength of carbon fiber in a composite and basic properties of a composite such as interlaminar shear strength (ILSS), compressive strength and bending strength are not sufficiently improved. Although various proposals of means for improving the basic properties of a composite, such as one comprising improving the surface properties of carbon fiber by electrolytic surface treatment, one comprising improving the properties of the matrix resin used and one comprising devising the arrangement of the carbon fiber constituting a composite have been made, no satisfactory results have been attained for the present.
Against such prior art technical backgrounds, the inventors of this invention have comprehensively studied means for improving the basic properties of a composite of carbon fiber with resin, whereupon they have found out that, as will be described in the detailed description of the invention hereinbelow, it is very effective to use a carbon fiber having a nonspherical section as well as to improve the internal structure of carbon fiber itself.
Such techniques of giving a noncircular section to carbon fiber are heretofore well known concerning pitch-derived carbon fiber as described in for example Japanese patent application Kokai publications No. 61-6313, No. 62-117821, No. 62-231024 and No. 62-131034. However, the non-circular cross-sections proposed by these disclosed techniques are those intended as means for improving a pitch-derived carbon fiber whose strength and modulus are low due to the heterogeneous crystalline structure as a lamellar structure inherent therein. Although the heterogeneous crystalline structure is decreased in amount, it still remains inside carbon fiber, so that the techniques are not those for improving the basic properties of a composite.
Further, an example of forming a nonspherical cross-section carbon fiber from a noncircular cross-section polyacrylonitrile (PAN) fiber obtained by the melt spinning process as a precursor is described in the Preprints of the 20th International SAMPE Technical Conference (1988). In order to make PAN melt-spinnable, one is obliged to adopt either means comprising adding a plasticizer to the polymer or means comprising the use of a low-MW PAN polymer, so that it is difficult in either case to give a precursor of a high degree of orientation. Therefore, it is almost impossible to improve the mechanical properties of a carbon fiber itself, even when this precursor is made into carbon fiber, and to improve the basic properties of a composite by using this carbon fiber. Especially in the means comprising adding a plasticizer to the polymer, this plasticizer functions as an impurity to adversely affect the properties of a carbon fiber, so that it is almost impossible to attain high mechanical properties.
Further, a polygonal cross-section carbon fiber produced from a polygonal cross-section PAN fiber as a precursor obtained by the wet spinning process is disclosed in Japanese patent application Kokai publication No. 57-42927. However, the polygonal cross-section carbon fiber thus produced cannot give a laminate having an increased availability of the strength of the fiber, so that it has a drawback that the basic properties of a composite such as tensile strength and compressive strength cannot be sufficiently increased.